1. Field of the Invention
The present invention relates generally to a method of measuring a dielectric constant of a printed circuit board for a Rambus inline memory module, and more particularly to a method of measuring a dielectric constant of a printed circuit board for a Rambus inline memory module in a frequency band of 400 MHz using time domain reflectometry without using a propagation velocity test coupon.
2. Description of the Prior Art
Printed Circuit Boards (PCBs) generally serve to easily connect various electronic devices to each other thereon in certain forms. PCBs are parts widely used in electronic products ranging from home appliances, such as digital televisions, to the newest communication devices. PCBs are classified into multipurpose PCBs, modular PCBs and packaged PCBs.
As electronic products and communication devices recently tend to be light, thin, and small and a Dynamic Random Access Memory (DRAM) market is thriving, the modular PCBs and the packaged PCBs have gained popularity. Additionally, memory manufacturers concentrate their efforts on facility investments and technology research and development to provide for their competitiveness in the DRAM market. Accordingly, PCB manufacturers focus their efforts on technology research and development and the mass production of modular PCBs and packaged PCBs for Rambus DRAMs (RDRAMs) to support the increased marketing of the RDRAMs. Therefore, PCB products are distributed into modular PCBs, Ball Grid Arrays (BGA) and Rambus Inline Memory Module (RIMM).
Meanwhile, modular PCBs equipped with Rambus chips malfunction due to a difference in Time Propagation Delay (TPD) between a data bus and a clock signal at a frequency band of 400 MHz. In this case, the TPD varies with the dielectric constant of a PCB, so it is necessary to measure the dielectric constant of the PCB.
A conventional method of measuring the dielectric constant of a PCB is described with reference to FIGS. 1 to 3 below.
FIG. 1 shows a Propagation Velocity (PV) test coupon used to measure the dielectric constant of a PCB according to the conventional method. FIG. 2 is a graph showing results of probing the PV test coupon using Time Domain Reflectometry (TDR) according to the conventional method. FIG. 3 is a flowchart showing the conventional method of measuring the dielectric constant of a PCB using the PV test coupon.
As shown in FIGS. 1 to 3, a PV test coupon 10 is fabricated in advance as a coupon used to measure the propagation velocity of a medium at step S11. Thereafter, the fabricated PV test coupon 10 is positioned on a work panel and the dielectric constant is measured at step S12.
As shown in FIG. 2, the short trace 21 and long trace 22 of the PV test coupon 10 are probed using the TDR. Thereafter, in order to measure a difference in TPD between positions “A” and “B” of FIG. 2, the TPD value of the position “B” is measured at step S13, and then the TPD value of the position “A” is measured at step S14.
Accordingly, the TPD is calculated by subtracting the TPD value of the position “A” from the TPD value of the position “B” at step 515.
The dielectric constant of the PCB is measured by applying the calculated TPD to a dielectric constant calculation equation (dielectric constant=[TPD×29.981/length]2) at step S16.
In this case, the long trace 22 of the PV test coupon 10 is 280.556 mm, and the short trace 21 thereof is 76.726 mm, so the difference between the two is 203.83 mm.
In the meantime, equipments, measurement conditions, and a test method are described below.
First, a TEK 11801A/B/C model of Tektronics Inc. is used as a digital sampling oscilloscope, and a TEK SD-24 model of Tektronics Inc. is used as a TDR sample head. In this case, a RIMM bare PCB measurement system is used as essential measuring equipment to measure the dielectric constant of a PCB, warm-up is performed for more than forty minutes before the measurement of the dielectric constant.
To obtain a graph shown in FIG. 2, the test conditions for probing a PV test coupon using the TDR are described in Table 1.
TABLE 1RISINGMAINVERTICALCURSORTIMETIME SCALESCALEAVERAGETYPE130 ps50 ps/div200 mp/div32paired(precision: 1 ps)dots
Under the test conditions described above, the dielectric constant of a PCB is measured as below.
“A” and “B” shown in FIG. 2 are positions on a time axis.
At the positions, the propagation velocity Vp of a medium is represented as Equation 1, as follows:                     Vp        =                              C                                          ɛ                r                                              =                      L            T                                              (        1        )            where C is the velocity of light, εr is the dielectric constant, L is length and T is time.
Accordingly, the dielectric constant εr is represented as Equation 2, as follows:                               ɛ          r                =                              [                                                                                T                    long                                    -                                      T                    short                                                                                        L                    long                                    -                                      L                    short                                                              ×              C                        ]                    2                                    (        2        )            where C is the velocity of light.
Accordingly, the dielectric constant εr is represented as equation 3, as follows:                               ɛ          r                =                              [                                                            B                  -                  A                                203.83                            ×              3              ×                              10                2                                      ]                    2                                    (        3        )            where the unit of B and A is ns.
However, the conventional method of measuring the dielectric constant of the PCB is problematic in that the length of an actual pattern is calculated and then a dielectric constant must be reversely calculated, or the PV test coupon must be manufactured and directly positioned on the work panel, so that the manufacturing efficiency of the PCB is reduced. Additionally, the dielectric constant of a PCB obtained by the conventional method is problematic in that an excessive error occurs between the obtained dielectric constant and a dielectric constant actually measured at a frequency band of 400 MHz.